In the production of solid oxide electrochemical cells it is necessary to chemically vapor deposit thin (typically 1 to 200 microns) oxide layers onto a porous tube or substrate sealed at one end. The oxide layer can be the ionically conducting electrolyte, typically yttria-stabilized zirconia, or the electronically conducting interconnect, typically magnesium-doped lanthanum chromite, which connects one cell to another.
In this modified chemical vapor deposition process, a first reactant containing a source of oxygen permeates through the pores in the substrate material and reacts with a metal halide gas on the other side of the substrate, depositing a metal oxide coating on the substrate. As the metal oxide reaction product grows on the substrate, it closes off the pores in the substrate strictly by chemical vapor deposition (CVD). The coating continues to grow by electrochemical vapor deposition (EVD) because oxygen ion transfer from the source of oxygen through the growing oxide layer takes place.
This process is very sensitive and it is often difficult to produce a satisfactory product. One problem is that metal halide vapors and by-products of the reaction, namely, chlorine or hydrochloric acid, can attack the substrate before a protective coating has been formed. In addition, cracks in the substrate and coating can develop as it is being deposited, due to the above-mentioned side reactions with the substrate, and these cracks cannot be easily sealed as the coating continues to grow. While stronger and more conductive ceramic materials have been found which could be used in the cell as substrates, these materials cannot now be used because they are more subject to deleterious chemical side reactions with halides and reaction by-products, and exposure time of the substrate to reactants must be minimized.